The invention generally relates to mobile communication networks and particularly to implementation of multiservice and multimedia calls in mobile communication systems.
In addition to conventional speech transmission, modern mobile communication systems offer various data transmission features to subscribers. The services of mobile communication systems are generally classified into tele-services and bearer services. A bearer service is a telecommunications service which provides signal transmission between the user interfaces and the network interfaces. An example of bearer services is a modem service. In the tele-service terminal services are also offered by the network. Important tele-services include speech, telefax and videotex services. The bearer services are usually subdivided into groups, such as asynchronous bearer services and synchronous bearer services, on the basis of a certain feature. In the case of the asynchronous bearer service the transmitting terminal and the receiving terminal are able to maintain their synchronization only for each single character to be transmitted. In the case of the synchronous bearer service the transmitting terminal and the receiving terminal are synchronized with each other for the whole duration of data transmission. Each such bearer service group comprises a number of bearer services, such as a transparent service and a non-transparent service. In the transparent service the data to be transmitted is unstructured and transmission errors are corrected only by means of channel coding. In the non-transparent service the data to be transmitted is structured into protocol data units (PDU) and transmission errors are corrected using automatic retransmission protocols (in addition to channel coding).
Existing mobile communication systems do not offer any special bearer services for multimedia calls or for simultaneous use of multiple data services. There is only one traffic channel available for a data call, the channel being either transparent (T) or non-transparent (NT). Depending on the required transmission rate a traffic channel may consist of one sub-channel (e.g. TDMA time slot) or several sub-channels (e.g. several TDMA time slots for high-speed data transmission, such as HSCSD in the GSM system). Any shared use of the traffic channel has to be implemented on the application layer, i.e. in the end user""s applications. Time-critical multimedia calls, e.g. video phoning, have to use transparent circuit-switched bearer services because other data services cannot guarantee as small variation of the transmission delay as the video service requires. Too long a transmission delay causes visible interference in the video image at the receiving end. Applications which are not time-critical and require accurate transmission usually use non-transparent bearer services. An example of such an application is transfer of data files.
FIG. 2 illustrates a conventional video call which is connected through a GSM mobile communication system (Global System for Mobile Communication). The point-to-point transmission link between the mobile stations MS1 and MS2 consists of a first transparent traffic channel between the MS1 and an interworking function IWF, which is located in a mobile services switching centre MSC, and of a second transparent traffic channel between the MS2 and the IWF.
The European Telecommunications Standard Institute (ETSI) has begun to define an application programming interface and environment (Mobile API/Mobile Application Programming Interface MEXE/Mobile Execution Environment) which provide an opportunity e.g. for operators and parties independent of mobile station producers to develop applications and services for mobile users. This development work also comprises a multiplexed interface between the terminal equipment (TE) and the mobile station MS. This multiplexed interface comprises a multiplexed protocol and enables simultaneous use of several activities and/or services between the TE and the MS through one serial link. Examples of suitable services are data services and short message services. A TE which is to use simultaneously both the data service and the short message service activates multiplexing in which the data of both services are multiplexed into one data stream and supplied to the mobile station MS through the serial link. The MS carries out a similar multiplexing procedure in which the short message data is separated from the data service data and the data is transmitted to the corresponding units in the mobile station MS. After this the MS transmits the data service data, e.g. speed, telefax or data file, over the allocated transparent or non-transparent traffic channel in the usual manner. The short message data is transmitted outside the traffic channel, as is defined e.g. in the GSM system. Other activities, such as GPRS and USSD which use transmission outside the GSM traffic channel and e.g. information on the battery status, configuration information, phone books, etc., can also be transmitted over the TE-MS interface in the same way as short messages MSS. Thus this arrangement does not improve implementation of multimedia calls and multiservice calls in a mobile communication network, either.
As was stated above, the problem related to the present mobile communication systems is that they either provide a transparent or non-transparent traffic channel or a packet service (such as GPRS, General Packet Radio Service) for a multimedia call between two mobile stations or between a mobile station and a terminal or server in a fixed network. Packet radio services and the non-transparent traffic channel are not suitable for video phoning or other time-critical applications. On the application layer the transparent bearer service requires an error correction protocol which is usually not optimized for a radio connection. This means that a multiservice and/or multimedia channel has to always use a transparent bearer service and perform multiplexing and error correction on the application layer in the end users""terminals.
Another problem that may arise concerns rate adaptation between the fixed network traffic channel and the transparent mobile network traffic channel. The mobile network traffic channel and the fixed network traffic channel may have different bit rates, e.g. 28.8 kbits/s (2*14.4 kbits/s) on the GSM traffic channel and 32 kbits/s on the fixed network traffic channel, which is in accordance with the ITU-T H.221 recommendation (Telecommunication Standardisation Sector of International Telecommunication Union). The different rates of the traffic channels cause an unbearable situation on the transparent point-to-point traffic channel because the bit rate has to be constant over the whole point-to-point connection. The GSM recommendations teach to use a GSM channel with a higher rate and fill the excess capacity with a useless fill. On the GSM sub-connection it is possible to use for example a traffic channel of 38.4 kbits/s, of which 32 kbits/s are user data and 6.4. kbits/s fill data, and a fixed network sub-connection of 32 kbits/s on which 32 kbits/s of user data are transmitted. This prior art method, however, wastes valuable radio channel resources.
The objective of the invention is to enable multimedia and multiservice calls in mobile communication systems in a simple manner, utilizing the radio channel resources effectively.
This is achieved with a method of transmitting two or more different information flows simultaneously in one call in a mobile communication network, the method being characterized in that it comprises the steps of: assigning one mobile network traffic channel to the call; dividing the assigned traffic channel into at least two sub-channels, and transmitting at least one information flow in each virtual sub-channel.
The invention also relates to methods of making a multiservice or multimedia call according to claims 2 and 10, a mobile station according to claim 11 and mobile communication networks according to claims 15 and 19.
According to the basic idea of the invention, one common traffic channel is allocated to a call, such as a multimedia or multiservice call, which requires simultaneous transmission of two or more different information flows apart from one another. Here the term traffic channel refers both to a single channel and to a set of two or more parallel sub-channels used in high-speed multichannel data transmission (e.g. the HSCSD channel of the GSM system). This common traffic channel is divided into at least two parts depending on the number of information flows that are to be transmitted separately, and each information flow is transmitted in a dedicated part of the traffic channel independently of the other flows. The transmitting end is provided with multiplexing means which multiplexes different information flows into a common traffic channel. The receiving end is provided with demultiplexing means which demultiplexes the signal received from the common traffic channel back into separate information flows. In a preferred embodiment of the invention at least one transparent information flow and at least one non-transparent information flow are multiplexed to the common traffic channel. Thus for example time-critical information which does not allow the use of an error correction protocol based on retransmission can be transmitted over the transparent sub-channel, and the non-transparent sub-channel can be used for transmitting less time-critical information which allows error correction based on retransmission. Two or more transparent signals (such as speech and video) may be multiplexed into one signal which is transmitted over the transparent sub-channel of the traffic channel. The non-transparent information flow may include e.g. data files. Several non-transparent signals may be multiplexed into one non-transparent data flow which is transmitted through the non-transparent part of the traffic channel.
The invention enables implementation of multiservice calls through the traditional mobile network traffic channel. The necessary multiplexing and demultiplexing are carried out in the terminals and interworking functions of the mobile communication network. Thus these functions do not need to be performed on the application layer by the end user like in prior art solutions. Thanks to the invention, one part of a multimedia or multiservice call can use a transparent bearer service and the other a non-transparent bearer service, whereas in the prior art the whole multimedia call had to use only the transparent bearer service.
Since the multiplexing and demultiplexing functions of the invention can be carried out in the mobile stations and interworking functions of the mobile network, the invention can be implemented without changes or with only minor changes in other parts of the network. In a preferred embodiment of the invention a transparent traffic channel is established first between two mobile stations or between a mobile station and an interworking function according to the normal call set-up procedure of the mobile communication network. After this the mobile stations or the mobile station and interworking function negotiate with each other using inband signalling about allocation of the transparent traffic channel to sub-channels. This inband negotiation can be implemented e.g. by means of a radio link protocol intended for a non-transparent traffic channel in the mobile communication system. When inband signalling is used, the rest of the mobile communication network does not necessarily need to support allocation of the traffic channel to different kind of data according to the invention in any way or be aware of this. In the case of a call between two mobile stations, a traditional transparent traffic channel connection can be established through the mobile communication network and the multplexing functions according to the invention carried out only in the mobile stations which also negotiate with each other about the allocation of the traffic channel through the traffic channel. Establishment of the traffic channel which is allocated to at least two sub-channels according to the invention can also be established by means of outband signalling, but this usually requires changes in the signalling messages, signalling procedures and network elements in different parts of the mobile communication network.
The present invention also provides a solution to the above-mentioned problem caused by the different bit rates of the mobile network transparent traffic channel and the fixed network transparent traffic channel. A transparent traffic channel whose bit rate R1 is close to the bit rate R2 of the fixed network transparent traffic channel is established between the mobile station and the interworking function. Considering effective utilization of the radio channel resources, it is preferable that R2  greater than R1, but the basic idea of the invention is also applicable to cases in which R1 =R2 and R1  greater than R2. After this equal shares of both the fixed network traffic channel and the mobile network traffic channel are allocated for transparent transmission, the bit rate of the shares being R3, when R3 less than R1 and R3 less than R2. Thus we can establish a point-to-point transparent connection which has a constant bit rate, i.e. R3. The remaining shares of the mobile network traffic channel and the fixed network traffic channel are allocated for non-transparent transmission. The bit rates R1xe2x88x92R3 and R2xe2x88x92R3 of the remaining shares may differ. However, the flow control and buffering in the interworking function according to the non-transparent protocol adjust the difference in bit rate and ensure data integrity in all situations. Thus the invention allows to avoid allocation of a traffic channel with an unnecessarily high speed to a transparent call to the fixed network and waste of excess capacity on transmission of fill data. Instead, the invention allows to set the bit rate of the transparent connection according to the need and use the excess capacity for non-transparent transmission. Furthermore, the invention enables multimedia and multiservice calls between the mobile station and the fixed network.